An analog-to-digital converter (ADC) is used in electronic devices for converting an analog signal (such as a voltage or a current) to a digital number that represents the analog signal's amplitude. Such a conversion involves quantization of an input signal via samples at periodic intervals. The result is a sequence of digital values that have converted a continuous-time and continuous-amplitude analog signal to a discrete-time and discrete-amplitude digital signal.
A successive-approximation ADC (SAR ADC) uses a comparator to successively narrow a range that contains the input signal (voltage in the remaining examples herein). At each successive step, the converter compares the input voltage to the output of an internal digital-to-analog converter (DAC) which might represent the midpoint of a selected voltage range. At each step in this process, the approximation is stored in a successive approximation register (SAR). In general, a SAR ADC converts a sample of an analog signal into a digital value in a specific number of clock cycles that is equal to the number of bits in the digital value along with a few cycles required to perform sampling. In this respect, SAR ADCs are medium speed. A faster speed for conversion is desired in faster speed circuits.